" In the old Persian mythology, the God of love, friendship, and light, the
God of Sun, Mithra (aka Mehr) was born in the longest night of the year .
This night is called Yalda which means the Birthday."

"In the west this time is marked as the shortest day of the year, Winter
Solstice. Mithraism spread to the Roman Empire. When Rome converted to
Christianity it borrowed some of its traditions from Mithraism:
"...Mithra's birth was announced as the night when Jesus Christ was born.
With it came all the festivities and rituals of Christmas. The Persian
(tree) Sarve was turned into the Christmas tree and so on."

Dr. Esmail Nooriala.

In ancients times, Iranians believed that MAZDA created "the day", a time of
production and work, and that AHRIMAN created "the night', a time of dark
secrets and wild attacking animals. The people believed that during the
longest night of the year, YALDA, the greatest war between the good and evil
forces was fought. The following day, when light shone over the dark night,
they knew that the days now would grow longer and they would enjoy more of
the sun.

During the night of YALDA, people sat around and celebrated snacking on the
remainder of dry and fresh fall fruits, and by waiting for the sunrise.

This celebration is still held today, among Iranians everywhere by lovers of
Iran and Iranian customs.